Delegate CommitSet#inject to #commits
Delegate CommitSet#inject to #commits

We use `#inject` in a few places, so delegate it to make it a little
easier to use.

Change-Id: Ic8fc8299f7307f2b4bae6021e1658bf536f71362
Reviewed-on: https://gerrit.causes.com/26642
Reviewed-by: Greg Hurrell <79e2475f81a6317276bf7cbb3958b20d289b78df@causes.com>
Tested-by: Greg Hurrell <79e2475f81a6317276bf7cbb3958b20d289b78df@causes.com>

causes/twigg,causes/twigg